GAZA, June 29 (Reuters) - A spokesman for gunmen in the Gaza Strip said they had fired a rocket tipped with a chemical warhead at Israel early on Thursday.

The Israeli army had no immediate comment on the claim by the spokesman from the al-Aqsa Martyrs Brigades, an armed wing of Palestinian President Mahmoud Abbas's Fatah movement.

The group had recently claimed to possess about 20 biological warheads for the makeshift rockets commonly fired from Gaza at Israeli towns. This was the first time the group had claimed firing such a rocket.

"The al-Aqsa Brigades have fired one rocket with a chemical warhead" at southern Israel, Abu Qusai, a spokesman for the group, said in Gaza.

An Israeli military spokeswoman said the army had not detected that any such rocket was fired, nor was there any report of such a weapon hitting Israel.